This application is based on French Patent Application No. 01 12 158 filed Sep. 20, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to an optical fiber cable with a high density of fibers, of the type which includes a tubular jacket surrounding a plurality of protection tubes in which groups of optical fibers are housed. It relates more particularly to a cable comprising a large number of fibers, in which the fiber protection tubes are disposed in layers and helically within each layer.
2. Description of the Prior Art
Optical fiber cables of the type referred to above conventionally include protection tubes for groups of optical fibers, the tubes having a cross section slightly larger than would be sufficient to accommodate the fibers of a group so that the fibers have some freedom of movement within the tubes in the event of deformation of the cable. This is known in the art. Combined with winding the tubes in a helix, this limits the longitudinal forces, and in particular the traction or compression forces, to which the optical fibers of a cable are subjected when the cable is spooled, unwound from a spool and/or bent, for example when spooling it for storage or when installing it. These operations must be carried out without exceeding predetermined mechanical limits for a cable with given specifications, because of the fragile nature of the fibers. This is also known in the art.
Thus research into optical fiber cables is currently being undertaken with a view to obtaining the greatest possible density of fibers in each cable to meet present and future demand for optical transmission of information using fibers as the physical transmission medium, whilst conforming to the mechanical constraints imposed by use of such fibers in the cable, especially during installation. The cables are ordinarily installed in conduits, underground or in a technical tunnel. One field of research aims to increase the number of fibers for the same outside diameter of the cable and/or to reduce the diameter of a cable accommodating a given number of fibers.
The invention therefore proposes an optical fiber cable including a tubular jacket which surrounds a plurality of protection tubes disposed in layers and in a helix within each layer, wherein the optical fibers are accommodated and have a relative freedom of movement.
According to one feature of the invention the cable includes at least one layer made up of an assembly of tubes which have an outside diameter smaller than that of the tubes of a layer that they surround, although the tubes of the two layers each contain the same number of optical fibers, to increase the capacity of the cable and/or to limit its outside diameter relative to a cable having layers in which the tubes have the same outside diameter.
According to the invention the cable includes at least one layer made up of an assembly of tubes having an outside diameter smaller than that of the tubes of the layer that they surround, the ratios between the inside diameter and the helical winding pitch for these layers being chosen to obtain an equivalent freedom of movement for the fibers regardless of the layer.
According to the invention the same ratio between the inside diameter and the outside diameter of the protector tubes is chosen for the tubes of the various layers.
In one embodiment of the invention, for use when the cable includes fibers having different sensitivities to any significant deformation, said fibers are distributed between the layers according to their sensitivity, the tubes containing the more sensitive fibers forming one or more layers surrounded by at least one layer in which the tubes contain less sensitive fibers.
In a different embodiment of the invention the tubes containing the fibers are assembled in a regular manner to form contiguous layers between the sheathing and a central bearing member around which the layers are placed. The layer adjoining the bearing member is made up of tubes having the same outside diameter at least equal to that of the tubes of the other layers, the layer adjoining the inside wall of the tube that forms the sheathing is made up of tubes having the same outside diameter smaller than that of the tubes of at least the layer adjoining the bearing member, and the respective numbers of tubes in the various layers are chosen to increase from the layer adjoining the bearing member to the layer adjoining the inside wall of the tube that forms the sheathing so that each layer is virtually continuous.
The invention, its features and its advantages are explained in the following description with reference to the figures mentioned below.